Here you will find links to online corpora that are suitable for English language learners.
The British National Corpus (BNC) is a 100 million word collection of samples of written and spoken language from a wide range of sources, designed to represent a wide cross-section of current British English, both spoken and written. (Summary information copied from the BNC homepage.)
Michigan Corpus of Academic Spoken English, containing more than 160 transcripts with over 2 million words of text.
This website allows you to quickly and easily search more than 100 million words of text of American English from 1923 to the present, as found in TIME magazine. (Summary information copied from the corpus website.)
COCA is the only large, genre-balanced corpus of American English. COCA is probably the most widely-used corpus of English. (Summary information copied from the corpus website.)
This is a very basic concordancer A concordancer is an app that allows you to analyse words or phrases as their context in a corpus. (on this website) that allows you to analyse corpora (texts) that you have compiled yourself.